This invention relates generally to the art of measuring physical parameters such as temperature by optical techniques, and, more specifically, to implementations that detect a time rate of decay of luminescence that is proportional to the temperature or other parameter being measured.
Optical techniques using luminescent sensors to measure temperature have in recent years become accepted for many applications and several commercial products have appeared. Some temperature dependent characteristic of the luminescent emission is measured and the temperature of the sensor determined from that measurement. The temperature of a surface can be measured by making direct contact between such a small luminescent sensor and the surface, or by applying the luminescent material directly on the surface, and then detecting the temperature-dependent luminescence by some type of remote or contact optical system. Such an optical system can use a length of optical fiber as one optical element. This technique has particular application for measuring the temperature of difficult to contact surfaces such as the surface of a rotating piece of machinery.
Commercially available products often attach the luminescent sensor at an end of a length of optical fiber to form a temperature-sensing probe. The temperature sensing probe is then placed in contact with an object, or within an environment, whose temperature is to be measured. The other end of the optical fiber is then connected to a measuring instrument. Since neither the sensor nor the optical fiber are sensitive to electrical or magnetic fields, temperatures may be measured in environments that are hostile to other measuring systems, such as in high voltage fields, intense electromagnetic radiation fields or beams, or environments which contain chemicals which might corrode electrical sensors or their metallic leads. Optical fiber probes may also be made to be implanted in the human body for measuring internal body fluid or tissue temperatures. Since the optical fibers do not conduct electricity, the probes can be used in the processing of flammable or explosive materials where electrically safe sensors are required. Also, since the sensors have very small mass and since the fibers do not conduct heat away from the measurement location, more accurate sensing is possible. Further, such fiberoptic probes may be permanently installed in large, expensive electrical equipment, such as electrical generators and power transformers. The very small size of the sensor and optical fiber cable, as well as its immunity from environmental factors that prevent more conventional temperature sensing techniques from being used, contribute to a wide range of additional applications.
The temperature-dependent characteristic of luminescence that is emerging as preferred for use in commercial systems is the temperature sensitivity of its time rate of decay. Luminescent materials used as temperature sensors, in response to a pulse of radiation that causes them to commence luminescence, exhibit a decay of their luminescence, after termination of the exciting pulse, with a rate which varies with temperature. The most desirable luminescent materials for such use exhibit an exponential decay of luminescent intensity after cessation of the excitation radiation. This then allows the temperature being measured to be correlated with a decay "time constant" of the luminescence, a quantity normally referenced as .tau., which is defined as the time to reach 1/e of the initially measured value of the decaying luminescence, where "e" is the natural logarithm base 2.71828.
Luminescence decay time instrumentation has generally used analog electronic circuits for signal processing. More recently, digital techniques have been employed. An example of a digital system is described in U.S. Pat. No. 5,107,445 of Jensen et al., assigned to Luxtron Corporation, assignee of the present application. This patent disclosure is expressly incorporated herein by this reference. In addition to using digital signal processing techniques, this patent describes forming on a single printed circuit card or board, the entire optical and electronic system for determining the decay time of a luminescence based sensor that is optically coupled with the board.
It is an object of the present invention to improve the structure and operation of the electronic system disclosed in the previously mentioned U.S. Pat. No. 5,107,445.
In such digital systems, an input signal must generally be acquired and amplified before the signal is digitally sampled in order to perform the digital processing. It is, therefore, a more specific object of the present invention to provide an improved "front end" analog system for use in such a digital processing system.
It is a further object of the present invention to adapt such a system to process signals from multiple sensors without making either the system or its operation significantly more complex.
It is a more general object of the present invention to provide a luminescent sensor-based measurement system that is usable with confidence over a long period of time to provide accurate results with consistency.
It is another general object of the present invention to provide a luminescent sensor-based measuring system that is small and compact, has low power requirements, and exhibits a high degree of ruggedness, for use in many different specific applications.